Silicon photonics multicarrier optical transceiver

ABSTRACT

Disclosed herein are techniques, methods, structures and apparatus that provide a silicon photonics multicarrier optical transceiver wherein both the transmitter and receiver are integrated on a single silicon chip and which generates a plurality of carriers through the effect of an on-chip modulator, amplifies the optical power of the carriers through the effect of an off-chip amplifier, and generates M orthogonal sets of carriers through the effect of an on-chip basis former.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of U.S. Provisional Patent Application Ser. No. 61/646,517 filed May 14, 2012 which is incorporated by reference in its entirety as if set forth at length herein.

TECHNICAL FIELD

This disclosure relates generally to the field of optical communications and in particular to techniques, methods and apparatus pertaining to silicon photonics multicarrier coherent transceiver that allows for higher data rates without requiring higher speed analog-to-digital converters (ADCs) and digital-to-analog converters (DACs) or higher order constellations.

BACKGROUND

Contemporary optical communications and other systems require reliable transceivers exhibiting high data rates. Consequently, methods, structures or techniques that facilitate the development or improvement of such transceivers—particularly those that do not require higher speed ADCs or DACs—would represent a welcome addition to the art.

SUMMARY

An advance in the art is made according to an aspect of the present disclosure directed to a silicon photonics multicarrier coherent transceiver wherein both receiver and transmitter are integrated onto the same silicon substrate.

Viewed from a first aspect, the present disclosure is directed to a silicon photonics multicarrier coherent transceiver that is integrated onto a single silicon substrate and employs a single laser—that is advantageously either off-chip or integrated on-chip using a gain chip. Advantageously transceivers according to the present disclosure generate a plurality of carriers using on-chip modulators, employ off-chip amplifiers to boost optical power, and uses basis former(s) to generate M orthogonal sets of carriers. Of particular advantage is the use of a basis former that includes a power splitter connected to M waveguides of different length.

BRIEF DESCRIPTION OF THE DRAWING

A more complete understanding of the present disclosure may be realized by reference to the accompanying drawings in which:

FIG. 1 shows a schematic top-view of a multi-carrier coherent transceiver photonics integrated circuit according to an aspect of the present disclosure;

FIGS. 2(a) and 2(b) shows a schematic of basis formers according to an aspect of the present disclosure wherein 2(a) depicts a demultiplexer each output having one carrier while 2(b) includes M arms of different path lengths connected to a power splitter, each output having all three carriers but with relative phase shifts of ±120° between them such that the three sets are mutually orthogonal;

FIG. 3 shows another schematic configuration of an alternative multi-carrier coherent transceiver photonics integrated circuit according to an aspect of the present disclosure; and

FIG. 4 shows another schematic configuration of an alternative multi-carrier coherent transceiver photonics integrated circuit according to an aspect of the present disclosure.

DETAILED DESCRIPTION

The following merely illustrates the principles of the disclosure. It will thus be appreciated that those skilled in the art will be able to devise various arrangements which, although not explicitly described or shown herein, embody the principles of the disclosure and are included within its spirit and scope.

Furthermore, all examples and conditional language recited herein are principally intended expressly to be only for pedagogical purposes to aid the reader in understanding the principles of the disclosure and the concepts contributed by the inventor(s) to furthering the art, and are to be construed as being without limitation to such specifically recited examples and conditions.

Moreover, all statements herein reciting principles, aspects, and embodiments of the disclosure, as well as specific examples thereof, are intended to encompass both structural and functional equivalents thereof. Additionally, it is intended that such equivalents include both currently-known equivalents as well as equivalents developed in the future, i.e., any elements developed that perform the same function, regardless of structure.

Thus, for example, it will be appreciated by those skilled in the art that the diagrams herein represent conceptual views of illustrative structures embodying the principles of the invention.

In addition, it will be appreciated by those skilled in art that any flow charts, flow diagrams, state transition diagrams, pseudocode, and the like represent various processes which may be substantially represented in computer readable medium and so executed by a computer or processor, whether or not such computer or processor is explicitly shown.

In the claims hereof any element expressed as a means for performing a specified function is intended to encompass any way of performing that function including, for example, a) a combination of circuit elements which performs that function or b) software in any form, including, therefore, firmware, microcode or the like, combined with appropriate circuitry for executing that software to perform the function. The invention as defined by such claims resides in the fact that the functionalities provided by the various recited means are combined and brought together in the manner which the claims call for. Applicant thus regards any means which can provide those functionalities as equivalent as those shown herein. Finally, and unless otherwise explicitly specified herein, the drawings are not drawn to scale.

Thus, for example, it will be appreciated by those skilled in the art that the diagrams herein represent conceptual views of illustrative structures embodying the principles of the disclosure.

By way of some additional background, it is noted that previous demonstrations of multicarrier coherent transceivers have been made. For example, R. Nagarajan has described in a paper entitled “10 Chanel, 100 Gbit/s per Channel, Dual Polarization, Coherent QPSK, Monolithic InP Receiver Photonic Integrated Circuit” which was presented at the Optical Fiber Communication Conference, 2011, p. OML7, however devices such as those described therein include 10 integrated lasers. Those skilled in the art will quickly recognize the difficulty associated with making such devices with high yield on a single chip and in particular where they all must be wavelength stabilized. Additionally, the devices disclosed therein use InP as the integration platform which is difficult to fabricate in high yield. Furthermore, the devices disclosed therein use separate chips for transmit and receive which requires additional packaging. Finally, devices such as those disclosed by Nagarajan may exhibit a poor spectral efficiency as they are limited by their wavelength stability to a relatively wide wavelength separation and such devices may have channels that are relatively narrow-band and thus susceptible to nonlinearities.

Turning now to FIG. 1, there is shown a schematic top-view of a multicarrier coherent transceiver PIC according to an aspect of the present disclosure. As depicted in FIG. 1, N=M=3; PBSR=polarization splitter and rotator; ITLA—integrated tunable laser assembly; PM-EDFA-polarization maintaining Er-doped fiber amplifier; and PD=photodetector.

Operationally, an integrated tunable laser assembly (ITLA) generates a narrow-linewidth continuous-wave laser (optical) signal. The generated optical signal enters a silicon PIC (SiPhPIC) where it is directed to a phase modulator which is preferably driven sinusodially at a frequency f. As may be appreciated, a single phase modulator may readily generate two sidebands exhibiting the same height as the carrier, resulting in three carriers. As may be further appreciated, the phase modulator may be replaced or substituted by an amplitude modulator or a combination of an amplitude modulator and a phase modulator.

Advantageously, other modulator schemes known in the art may be used to generate multiple carriers. More particularly, and by way of example, a modulator positioned within a ring resonator may be used to generate multiple carriers. Yet another exemplary alternative may include an arrangement of single-sideband modulators.

For the purposes of this discussion and as depicted in FIG. 1, we let N be the total number of carriers exiting the modulator assembly. Typically, these N carriers do not exhibit enough optical power to supply both the modulator inputs in the transmitter and the local oscillators in the receiver. As a result—and according to an aspect of the present disclosure—the N carriers are directed from the PIC to an off-chip, Er-doped fiber amplifier (EDFA) where they are amplified through the effect of the EDFA.

As may be appreciated, it is preferred that the EDFA be polarization maintaining, however this is not absolutely necessary. Alternatively, a semiconductor gain element known in the art may be employed wherein that gain element is attached directly to the PIC or coupled via fiber coupler.

The amplified carriers are then directed from the EDFA to the PIC where they enter a “basis former”. The basis former splits the N carriers into M orthogonal sets, where M<=N.

Two examples of basis formers according to an aspect of the present disclosure for three carriers (N=3) are shown schematically in FIGS. 2(a) and 2(b). As depicted in FIG. 2(a), the basis former comprises a wavelength demultiplexer wherein each output is a separate one of the three carriers. Such a basis former may be constructed from an arrayed waveguide grating, Mach-Zehnder filters, or ring resonator filters. FIG. 2(b) shows an alternative basis former according to an aspect of the present disclosure. More specifically, the alternative embodiment shown in FIG. 2(b) comprises a 1×M power splitter followed by M waveguides of linearly increasing path length. In an exemplary embodiment, the path length difference between successive waveguides is defined by c/(fn_(g)M), where c is the speed of light in a vacuum and n_(g) is the waveguide group index.

One appreciable advantage of this configuration shown in FIG. 2(b) is that it does not require filters to remain constant despite temperature changes, as is the case with the configuration shown in FIG. 2(a). Additionally, data for each “channel” is spread over the entire signal spectrum, thereby reducing effects of nonlinearities.

With continued reference to FIG. 1, it may be observed that outputs of the basis former are directed to a number of quadrature phase-shift keying (QPSK) modulators. As depicted therein, the number of QPSK modulators is 2N. The outputs of the modulators are subsequently combined by power converter(s). One half of the modulated outputs are polarization rotated, and then further combined through the effect of polarization splitter and rotator and subsequently output as combined Tx output signal.

Notably, modulators other than the QPSK modulators shown are contemplated according to the present disclosure. More particularly, 8-quadrature amplitude modulation (QAM) modulators may be employed equally well. One constraint on the modulator(s) employed however is that the modulator symbol rate should equal to or be less than f (as defined above).

Advantageously, the polarization splitter and rotator (PBSK) may comprise a 2D grating coupler for example. When such a 2D grating coupler is employed, explicit polarization rotators are not required and instead the 2D grating coupler combines the co-polarized inputs into a single, polarization multiplexed output.

With respect to receiver function(s), received signals (Rx input) are split through the effect of a PBSR and the resulting portions are directed into 90 degree hybrids where they are combined with some of the outputs from the basis former which act as local oscillators. Optical signals output from the hybrids are detected by photodetectors and directed to 4N analog to digital converters for subsequent processing.

Turning now to FIG. 3, there it shows an alternative embodiment of a PIC according to an aspect of the present disclosure. As shown in FIG. 3, a number of additional basis formers are employed. Using the example of N=3 carriers, each of the individual basis formers outputs the N carriers which are then applied to an individual QPSK modulator or 90 degree hybrid as shown. A particular advantage of this exemplary configuration is that there are fewer waveguide crossings on the PIC as compared with the PIC shown in FIG. 1.

Turning now to FIG. 4, there is shown yet another PIC according to an aspect of the present disclosure. More specifically, the PIC shown in FIG. 4 includes an integrated CW laser.

Those skilled in the art will readily appreciate that while the methods, techniques and structures according to the present disclosure have been described with respect to particular implementations and/or embodiments, those skilled in the art will recognize that the disclosure is not so limited. More particularly, the variations depicted in the FIGUREs may be combined as appropriate. For example, the integrated laser of FIG. 4 may be included in the arrangement of FIG. 3 or FIG. 1. Accordingly, the scope of the disclosure should only be limited by the claims appended hereto. 

The invention claimed is:
 1. A multicarrier coherent optical transceiver comprising: a silicon substrate; an optical transmitter integrated on the silicon substrate and comprising a plurality of transmitter modulators; an optical receiver integrated on the silicon substrate and comprising a plurality of photodetectors; and a carrier generator comprising a signal modulator integrated on the silicon substrate and optically coupled to the optical transmitter and the optical receiver, the signal modulator being configured to receive as input an optical signal having an input carrier from an optical signal source and to generate, from the input carrier, a plurality of output carriers including a first output carrier and a second output carrier; and a wavelength demultiplexer integrated on the silicon substrate, coupled to the carrier generator, and configured to spatially separate the plurality of output carriers; wherein the plurality of transmitter modulators of the optical transmitter integrated on the silicon substrate includes a first transmitter modulator configured to receive the first output carrier and a second transmitter modulator configured to receive the second output carrier from the wavelength demultiplexer, and the plurality of photodetectors of the optical receiver includes a first photodetector configured to receive the first output carrier and a second photodetector configured to receive the second output carrier from the wavelength demultiplexer.
 2. The multicarrier coherent optical transceiver of claim 1, wherein the plurality of transmitter modulators comprise at least one quadrature phase shift key (QPSK) modulator.
 3. The multicarrier coherent optical transceiver of claim 1, wherein the signal modulator comprises a phase modulator.
 4. The multicarrier coherent optical transceiver of claim 1, wherein the signal modulator comprises an amplitude modulator.
 5. The multicarrier coherent optical transceiver of claim 1, further comprising an optical amplifier configured to amplify the plurality of output carriers.
 6. The multicarrier coherent optical transceiver of claim 5, wherein the optical amplifier is an Erbium-doped fiber amplifier.
 7. The multicarrier coherent optical transceiver of claim 5, wherein the optical amplifier is disposed on a second substrate other than the silicon substrate.
 8. The multicarrier coherent optical transceiver of claim 5, wherein the optical amplifier is positioned optically between the carrier generator and the plurality of transmitter modulators.
 9. The multicarrier coherent optical transceiver of claim 1, wherein the optical transmitter further comprises a power combiner coupled to the plurality of transmitter modulators and configured to combine a plurality of output signals of the plurality of transmitter modulators onto an output waveguide.
 10. The multicarrier coherent optical transceiver of claim 1, wherein the optical receiver further comprises a polarization splitter having a first output waveguide optically coupled to the first photodetector of the plurality of photodetectors and a second output waveguide optically coupled to the second photodetector of the plurality of photodetectors, wherein the polarization splitter is configured to receive an input signal having a first polarization state and a second polarization state, different from the first polarization state, and to output a first signal having the first polarization state onto the first output waveguide and a second signal having the second polarization state onto the second output waveguide.
 11. The multicarrier coherent optical transceiver of claim 1, wherein the first photodetector is configured to beat the first output carrier with an input signal and the second photodetector is configured to beat the second output carrier with the input signal.
 12. The multicarrier coherent optical transceiver of claim 1, wherein the wavelength demultiplexer comprises a power splitter, integrated on the silicon substrate, having an input waveguide optically coupled to the carrier generator and a plurality of output waveguides, including a first output waveguide optically coupled to the first transmitter modulator and a second output waveguide optically coupled to the second transmitter modulator.
 13. The multicarrier coherent optical transceiver of claim 12, wherein the plurality of output waveguides includes the first output waveguide having a first length, the second output waveguide having a second length, and a third output waveguide having a third length, and wherein the first length, the second length, and the third length differ from each other and conform to a linearly increasing relationship.
 14. The multicarrier coherent optical transceiver of claim 13, wherein a path difference between two adjacent output waveguides of the plurality of output waveguides is defined by c/(fn_(g)M) where c is a speed of light in a vacuum, n_(g) is a waveguide group index, f is a frequency associated with an output carrier of the plurality of output carriers and M is a number of output waveguides of the plurality of output waveguides.
 15. The multicarrier coherent optical transceiver of claim 1, wherein the plurality of output carriers are mutually orthogonal.
 16. The multicarrier coherent optical transceiver of claim 1, wherein the signal modulator is optically coupled to the optical transmitter and the optical receiver through optical waveguides integrated on the silicon substrate. 